In recent years, attention has been directed to lithium secondary cells or batteries which are adapted for a greater capacity and higher energy density for use as power sources for electric motor vehicles or hybrid cars. For example, FIGS. 5 and 6 show a cylindrical lithium secondary cell which comprises a cylindrical cell can 1 having a cylinder 11 and lids 12, 12 welded to the respective ends thereof, and a rolled-up electrode unit 2 encased in the cell can 1. A pair of positive and negative electrode terminal assemblies 9, 9 are attached to the lids 12, 12, respectively. The rolled-up electrode unit 2 is connected to the terminal assemblies 9, 9 by a plurality of current collector tabs 3, whereby the electric power generated by the electrode unit 2 can be delivered to an external device from the pair of terminal assemblies 9, 9. Each lid 12 is provided with a gas vent plug 13.
With reference to FIG. 7, the rolled-up electrode unit 2 comprises a positive electrode 23 containing a lithium containing composite oxide, a negative electrode 21 containing a carbon material, and a separator 22 impregnated with a nonaqueous electrolyte and interposed between the electrodes, the assembly of these components 21 to 23 being rolled up into a cylinder. A plurality of current collector tabs 3 outwardly extend from each of the positive electrode 23 and the negative electrode 21 of the unit 2, and the outer ends 31 of the current collector tabs 3 of the same polarity are joined to one electrode terminal assembly 9. For convenience, sake, only some of these tabs are shown as being joined at their outer ends to the terminal assembly 9 in FIG. 6, while the outer ends of the other tabs connected to the assembly 9 are omitted from the illustration.
The electrode terminal assembly 9 comprises a screw member 91 extending through a hole in the lid 12 of the cell can 1 and mounted on the lid 12. The screw member 91 has a flange 92 at its base end. An insulating packing 93 is fitted in the hole of the lid 12 for electrical insulation and effective sealing. The screw member 91 has a washer 94 fitted therearound from outside the cylinder 11, and a first nut 95 and a second nut 96 screwed thereon similarly. The first nut 95 is tightened up to clamp the insulating packing 93 between the flange 92 of the screw member 91 and the washer 94 and thereby seal off the hole more effectively. The outer ends 31 of the current collector tabs 3 are secured to the flange 92 of the screw member 91 by laser welding or ultrasonic welding.
For connecting the current collector tab 3 to the negative electrode 21 or positive electrode 23 of the rolled-up electrode unit 2, a current collector strip forming the electrode and coated with an electrode material over a surface has a known structure comprising a portion of the surface not coated with the electrode material and having the base portion of the current collector tab secured thereto by laser welding or ultrasonic welding (JP-A No. 267528/1994).
When the nonaqueous electrolyte secondary cell described develops a short-circuit in its interior, a great current is likely to flow. To avoid such an incidence, an electrode structure has been proposed. For example, a positive electrode 8 comprises, as shown in FIG. 8, a current collector 81 which is provided with an electrode material 83 over each of its opposite surfaces, with a PTC (positive temperature coefficient) element layer 82 formed therebetween (JP-A No.220755/1995). The PTC element providing the layer 82 has a positive temperature coefficient of resistance, such that when a current in excess of a predetermined value flows therethrough, the electric resistance value of the element rapidly increases to exhibit a current suppressing effect. When the secondary cell having the proposed electrode structure develops an inside short-circuit, a current exceeding the predetermined value will not flow continuously.
However, in the case of the conventional nonaqueous electrolyte secondary cell having the PTC element layer 82 shown in FIG. 8, the presence of the PTC element layer 82 between the current collector 81 and the electrode material 83 makes the quantity of the electrode material 83 correspondingly smaller than otherwise per unit volume of the cell can, consequently entailing the problem of greatly reducing the discharge capacity per unit volume of the cell can, i.e. energy density.